Dynamic cable having improved properties and process and plant for manufacturing such a cable

ABSTRACT

A cable includes a plurality of strands around a core, where the space between the strands and/or between core and strands is filled with a polyurethane-based polymer material. The cable is produced by depositing the polymer material over the core and/or strands with a coating device a certain distance upstream of the cabling point. The polymer&#39;s composition and coating point are determined so that the strands are placed around the core when the polymerization of the compound is sufficiently advanced so that the polymer formed is in a viscoplastic state suitable to form an approximately continuous adherent coating layer, but still sufficiently fluid to insinuate into the interstices between core and strands and between the strands.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to lifting or traction cables, alsocalled dynamic cables as they normally work in tension, but which arealso subjected to bending under loads liable to vary greatly. Suchcables are in particular lifting or traction equipment cables wound ontothe drum of a winch.

[0002] Cables of the type considered here are conventionally formed byat least one ply of steel wire strands wound in a helix around a centralcore. The winding is carried out by means of a cabling head encirclingthe core, the head undergoing a rotation movement about itself andcontinuously depositing the strands on the running central core.

DESCRIPTION OF THE PRIOR ART

[0003] The characteristics required for the above-mentioned cables are,in particular, high flexibility, good diametral uniformity, ensuringuniform permanent elongation, and good abrasion resistance. For furtherexplanations about the required characteristics of such cables and theconstraints with which they have to comply, reference may be made forexample to the French patent application filed under No. 93/08648.

[0004] One type of cable normally used for the aforementioned purposesis a hybrid cable consisting of eight steel wire strands wound in ahelix around a core made of a natural textile based on hard fibers, suchas sisal. A similar cable is also known, but one in which the coreitself is hybrid, namely consisting of metal strands and textile fillingfibers. The dynamic cables may also be cables consisting of metalstrands over a metal core, which may or may not be independent. Forexample, cables are known which consist of twelve outer strands woundover an independent Warrington-type core, or else cables consisting ofnine outer strands over an independent metal core itself consisting ofnine strands.

[0005] The relatively low radii of curvature to which the cable isstrained in this type of application create large bending forcesthroughout the cable, but most especially generate substantialdifferences in paths between the various strands and also between theirconstituent wires. There are therefore repeated relative displacementsbetween any one strand and another, which may already cause local wearand elongation simply by rubbing. In addition, the curvature of thecable, when it is moreover under load, that is to say subjected totensile stresses, creates high transverse compressive forces between thewires in contact with one another, resulting in a loss of transversespeed of the cable and easily causes wear by indentations. Theseindentations end up by locally reducing the cross section of the wiresand are therefore detrimental to the strength and the lifetime of thecables. In addition, these reductions in cross section may promoteelongation, something which is particularly to be avoided inapplications of the lift cable type.

[0006] It is already known to use cables comprising sisal-type naturalor synthetic (polypropylene) fibers, the function of which is to form acushion inserted between the various plies of strands. The aim is thusto avoid, or at least limit, the rubbing of the wires or strands againstone another, and the resulting risks of indentation. One common aspectof these inserts made of non-metallic materials is that they do notcontribute directly to taking up the tension in the cable, that is tosay they do not contribute directly to increasing the tensile strengthof the cable.

[0007] It has already been proposed, in Document FR-2 783 585, to placebetween the core and the outer strands a polyamide sheath whose purposeis especially to form a layer distributing the compressive forcesexerted radially by the strands on the core that they surround. However,the effect of such a sheath is limited to the interface between core andstrands, and fitting the sheath poses problems during manufacture of thecable.

[0008] Cables are also known which comprise, between the core and theouter strands, a layer of a plastic, especially polyethylene. In a firstmanufacturing phase, the polyethylene is deposited over the core byextrusions so as to coat it. The core thus coated is rewound and storedawaiting use. Then, during cabling, the core is again unwound and theplastic sheath surrounding the core is heated in order to soften itsufficiently and allow the strands to be spiralled around the core, in aconventional manner, but by encrusting them in the plastic softened atthe cabling point. However, such a manufacturing process requiresseveral unwinding and rewinding operations and a not inconsiderableconsumption of energy needed to soften the plastic during cabling.

[0009] This expenditure on energy could be reduced by means of theprocess for coating plastic on cables proposed in Document FR-2 553 442,and in which process the core precoated with plastic is assembled withstrands which are themselves coated with plastic. The plastic isinjected onto the strands at the actual stranding device in such a waythat the coating and the assembling of the strands take placesimultaneously at the cabling point. However, such a cable manufacturingprocess still requires several unwinding and rewinding operations sincethe core must be plastic-coated beforehand so as to ensure the presenceof a protective layer between core and strands. In addition, theinjected plastic is in a viscous state when the strands are being coatedand it is only after assembly that it solidifies. However, during theassembling operation this relatively fluid material can flow, resultingin imbalances in the thickness of the protective layer lying between thestrands. In addition, the strands can slide over one another, thusthinning, or even removing, the layer of plastic which separates them.

SUMMARY OF THE INVENTION

[0010] The present invention is aimed at the manufacture of a compositecable having, in the free spaces between strands, a plastic at leastpartially filling these spaces, the cable obtained being particularlysuitable for use as a traction or lifting cable, that is to say, ingeneral, in all applications in which the cable has, in service, to besuccessively wound onto a drum or the like and unwound therefrom.

[0011] The aim of the invention is in particular to increase thelifetime of such a cable, especially by reducing the phenomenon of wearby mutual indentation of the strands. Its aim is also to allow economicmanufacture without substantial modifications to the existing cablingplants. Its aim is also to make it possible to deposit, in a singleoperation, a layer of plastic intended to be placed between core andstrands and between strands.

[0012] With these objectives in mind, the subject of the invention is aprocess for manufacturing a dynamic cable comprising a plurality ofstrands stranded around a core, the spaces between strands and/orbetween core and strands being at least partly filled with a polymermaterial, according to which process the cable is formed by depositing,around a running core, a plurality of strands wound in a helix, whichprocess is characterized in that a polymerizable compound is deposited,on the run, over the core and/or the strands at a coating point at acertain distance upstream from the cabling point in the direction inwhich the core runs, said deposited compound comprising a prepolymer anda polymerization accelerator, the composition of said compound and thedistance from the coating point being determined so that the strands arewound around the core when the polymerization that the depositedcompound is undergoing is sufficiently advanced so that the polymerformed is in a viscoplastic state, a coating layer which is pasty butstill sufficiently fluid to flow into the interstices between core andstrands, and between strands, and in that the polymerization iscompleted only when the cable, once formed, is wound onto the take-upreel.

[0013] A major advantage of the invention is that the integration ofsaid polymer material into the structure of the cable does not requireadditional operations since it is put in place simultaneously with thecabling operation, thereby making the process particularly economic.

[0014] According to a first method of implementation, the polymerizablecompound is deposited only over the core (by an injection headsurrounding the core).

[0015] According to an alternative arrangement, the polymerizablecompound is deposited by coating jointly the core and the strands, andthe possible surplus material on the surface of the cable after cablingis removed after the assembling head and before the cable is wound uponto the take-up reel. This arrangement allows all the intersticesbetween the core and the strands to be effectively filled.

[0016] Also preferably, the polymer material is polyurethane, the use ofwhich during cable manufacture is particularly conducive to goodpenetration between the strands, as will be seen later. In addition, itscost is moderate and therefore not an imposition on that of the cableobtained.

[0017] The polymerizable compound is in fact produced just beforecoating, by mixing a prepolymer containing isocyanates with apolymerization accelerator in predetermined proportions.

[0018] The properties of the components of the mixture produced for thecoating and the processing conditions must ensure that the depositedcompound will be, under the cable manufacturing conditions, especiallythe temperature and running speed conditions, in a sufficiently pastystate just after coating in order for it to adhere to the core and formthereon, right at the cabling point, a malleable, semi-solid, coatinglayer similar to modelling clay, and therefore capable of molding thestrands which are deposited thereon.

[0019] Its adhesion to the constituent wires and strands of the cablewill make it possible to fill, by flow, under the action of the pressureresulting from winding the outer strands over the core, the spaces notoccupied by said strands or the core, without any polymer materialundergoing polymerization flowing to the outside of the cable andsticking onto the components of the cabling machine, such as guiderollers or other members that have to be in contact with the cableformed.

[0020] Next, the polymerization must be completed after the cable isformed, for example when it is already wound on the take-up reel of thecabling machine, or at least in a geometrically sufficiently stablestate so that the state of polymerization, commonly called the “weakcuring” phase, takes place only when the outer strands are no longercapable of moving with respect to one another and with respect to thecore in the cable being manufactured.

[0021] The components of the mixture used must in fact be determined sothat the kinetics of the polymerization reaction comprise:

[0022] a first polymerization phase taking the mixture to theabovementioned pasty state, this first phase having to be short or evenvery short in order to ensure that the polymer is well held on the coreas soon as it is deposited thereon; and

[0023] a second, polymerization continuation phase in which the polymerremains pasty for as long as possible before reaching the weak curingphase, this taking into account, of course, the run speed of the cableduring its manufacture, in order to be able to complete the cablingoperation and freeze the relative position of the strands and of thecore, while leaving the strands to become encrusted in the polymermaterial without there being any risk of the polymer formed cracking.

[0024] Polyurethane has proved to be particularly suitable for meetingthe abovementioned constraints and objectives. Thus, polyurethanepractically fills all the spaces lying between core and strands, on theone hand by coating as close as possible the surface of the core, andeven by being inserted between the constituent wires or strands, as thecase may be of the outer layer of the core, and, on the other hand, bybeing extended between any two adjacent outer strands up to the pointwhere said strands are closest to one another, that is to say at theline along which said outer strands may be brought, during use, intopermanent or occasional contact with one another.

[0025] In the finished cable, each strand thus has its positiondetermined by the polyurethane which has polymerized in contact with it,making a kind of receiving trough specifically corresponding to theexternal dimensions and shapes of said strand. Consequently, the risksof undesirable contacting and rubbing between neighboring strands orbetween strands and core are considerably reduced.

[0026] In addition, not only are the strands held in their respectivetrough, as indicated above, but, because of the inclination of the wireswith respect to the longitudinal direction of the strands, each wire issimilarly held in place, at the outer surface of the strand, in its ownmini-trough, the dimensions of which are of course smaller than in thecase of the strands, but above all inclined with respect to thelongitudinal direction of the trough for the strands. This relativeinclination results in the strands not being able to slidelongitudinally in their respective troughs.

[0027] Thus, overall, not only are the strands held together and withrespect to the core in the direction perpendicular to the longitudinaldirection of the cable, but they are also held together in saidlongitudinal direction. In practice, there can therefore no longer beany relative sliding between strands in the longitudinal direction,thereby preventing rubbing and hence considerably limiting the wear ofthe constituent wires of the strands. In addition, the relative holdingof the strands together in the longitudinal direction ensures that theloads in service are distributed, which distribution may be more uniformthan when the strands are able to slide with respect to one another.

[0028] This is because, in the cables according to the prior art, therelative reptational movement of the strands over one another, combinedwith unequally distributed loads, may lead to over lengthening of someof the strands, this over lengthening possibly going as far as to causelocalized swelling of the cable, the overly long strand forming a kindof looped excrescence, also called “birdcage”, to the detriment of thestrength and durability of the cable. By preventing the strands fromsliding with respect to their polyurethane molded troughs, the inventionmakes it possible to avoid these problems by preventing any tendency ofthe strands to undergo relative displacement.

[0029] The flexibility of the cable, necessary for its dynamic operationinvolving successive deflections and straightenings when being wound upon drums or when passing over pulleys, is, however, maintained thanks tothe toughness and high level of elastic elongation of polyurethane,which allow it to follow the normal bending deformations of the cable inservice, even when combined with high tensile forces, without crackingor deteriorating in other ways. Thus, the inevitable microslippingbetween neighboring strands or wires during bending of the cable iseasily absorbed by the elasticity of the polyurethane, without anyreduction in cross section of the polyurethane taking place at theinterfaces between said strands or wires.

[0030] The subject of the invention is also a cabling plant comprisingmeans for making a core of a cable run and a cabling head for depositinga plurality of strands in a helix around the core, and comprisingcoating means for depositing a polymerizable compound over the coreand/or the strands, the plant being characterized in that said coatingmeans are at a certain distance upstream of the cabling head withrespect to the direction in which the core runs and in that theycomprise:

[0031] a prepolymer reservoir,

[0032] a polymerization accelerator reservoir and

[0033] an injection head with a mixing chamber connected to saidreservoirs,

[0034] said coating means being controlled so as to deposit, over therunning core and/or strands, a layer of said compound thus obtained bymixing.

[0035] According to one particular arrangement, said coating meanscomprise an injection head surrounding the core and controlled so as todeposit said layer only around the core.

[0036] The subject of the invention is also a cable, obtained accordingto the process mentioned above, comprising a plurality of strandsstranded around a core, wherein the spaces between core and strands andbetween wires are filled with a polymer material deposited at least overthe core and being insinuated between the strands and/or between theconstituent wires after cabling. Preferably, the polymer material ispolyurethane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Further features and advantages will become apparent from thedescription, which will be given by way of example, of a cable accordingto the invention and of its manufacturing plant, with reference to thesingle appended plate of drawings, in which:

[0038]FIG. 1 is a cross section through a cable according to theinvention;

[0039]FIG. 2 is a schematic representation of the manufacturing plant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The cable 9, shown in cross section in FIG. 1, comprises a core1, which is for example a metal core formed by assembling strands 5,which in this case are seven (1+6) in number, each being formed byseventeen stranded steel wires 6 in two plies over a central core(1+8+8). The nature and the construction of the core are, however, in noway a limitation of the invention. The core could also comprisecomposite strands.

[0041] The core 1, the boundaries of which are shown symbolically by thecircle 10, is surrounded by a single ply of strands 2 (in this caseeight in number), each formed by thirty-one steel wires 7 organized inthree plies (1+6+12+12). The strands 2 are wound in a spiral over a kindof polyurethane sheath 3 which grips the core, the strands being roughlyencrusted half way into said sheath. Incidentally, it will be noted thatthere is no bonding or adhesion particularly desirable between thestrands or their constituent wires and the polyurethane, the strandsbeing held in position with respect to said sheath essentially by thegeometrical interaction between the surface shapes of the strands andthe corresponding trough shapes in the mass of polyurethane, said shapeshaving been mentioned above. An ancillary advantage is that, due to thefact that such adhesion is not sought, there is no need for prioroperations on the core or the strands for their preparation, cleaning,degreasing, etc.

[0042] This cable is produced in a cabling plant as shown in FIG. 2.This plant includes means for making the core 1 and the cable formed runin the direction of the arrow F, these means comprising pay-out reels 11delivering the strands 5 constituting the core 1 and a take-up reel 12on which the cable 9 formed is wound.

[0043] A cabling head 13, of type known per se, ensures that the strands2 coming from respective pay-out reels shown schematically at 14 arewound in a helix around the core 1.

[0044] Placed in the path of the core, at a certain distance upstreamfrom the cabling head 13, for example about twenty centimeterstherefrom, is an injection head 15 which deposits, on the run, apolymerizable compound 3 over the core. Said polymerizable compound,based on polyurethane, is produced in the injection head 15 by mixing aprepolymer containing isocyanates with a polymerization accelerator.Other products having equivalent properties may also be suitable.

[0045] For this purpose, the injection head is provided with a mixingchamber connected to two reservoirs, namely a prepolymer reservoir and apolymerization accelerator reservoir. Flow control means are provided atthe outlet of said reservoirs. This is because it is necessary to beable to adjust the proportions of the various components of the mixtureso as to obtain the desired consistency of the polymer being formed atthe moment of assembling the cable.

[0046] Thus, with the abovementioned components, good results areobtained when the ratio by mass of the compounds introduced into theinjection head is 20 grams of polymerization accelerator per 100 gramsof prepolymer.

[0047] The polyurethane 3 is deposited over the core in the pasty state.If the polyurethane deposited is in excess, it will be wiped off by anysuitable means after it has left the cabling head. This may be the casewhen the construction of the cable results in the inter strand spacebeing relatively large, which space it is desired to fill, or whenintentionally the amount of compound deposited over the core isdetermined so as to be able to flow beyond said line of least distancebetween adjacent strands, in order to ensure, and be able to check, thatat least a peninsula 4 of polymer is present between the wires of twoadjacent strands, as may be seen, for example, in FIG. 1.

[0048] The invention is not limited to the cable and the process whichhave been described above solely by way of examples. In particular, thenumber and the composition of the strands, and the composition of thecore, could be modified.

1. A process for manufacturing a dynamic cable comprising a plurality ofstrands stranded around a core, the spaces between strands and/orbetween core and strands being at least partly filled with a polymermaterial, according to which process the cable is formed by depositing,around a running core, a plurality of strands wound in a helix, whereina polymerizable compound is deposited, on the run, over the core and/orthe strands at a coating point at a certain distance upstream from thecabling point in the direction in which the core runs, said compoundcomprising a prepolymer and a polymerization accelerator, thecomposition of said compound and the distance from the coating pointbeing determined so that the strands are wound around the core when thepolymerization that the deposited compound is undergoing is sufficientlyadvanced so that the polymer formed is in a viscoplastic state suitablefor forming a coating layer which is pasty but still sufficiently fluidto flow into the interstices between core and strands, and betweenstrands, and wherein the polymerization is completed only when the cableis formed.
 2. The process as claimed in claim 1, wherein thepolymerizable compound is deposited only over the core.
 3. The processas claimed in claim 1, wherein the polymerizable compound is depositedby coating jointly the core and the strands.
 4. The process as claimedin claim 1, wherein the polymer material is based on polyurethane. 5.The process as claimed in claim 1, wherein the polymerizable compound isproduced just before coating, by mixing a prepolymer containingisocyanates with a polymerization accelerator in predeterminedproportions.
 6. A cabling plant comprising means for making a core of acable run and a cabling head for depositing a plurality of strands in ahelix around the core, and comprising coating means for depositing apolymerizable compound over the core and/or the strands, wherein saidcoating means are at a certain distance upstream of the cabling headwith respect to the direction in which the core runs and wherein theycomprise: a prepolymer reservoir, a polymerization accelerator reservoirand an injection head with a mixing chamber connected to saidreservoirs, said coating means being controlled so as to deposit, overthe running core and/or strands, a layer of said compound thus obtainedby mixing.
 7. The cabling plant as claimed in claim 6, wherein saidcoating means comprise an injection head surrounding the core andcontrolled so as to deposit said layer over the core.
 8. A cablecomprising a plurality of strands cabled around a core, the spacesbetween core and strands being filled with a polyurethane-based polymermaterial, where in said material is a compound obtained by mixing aprepolymer with a polymerization accelerator and wherein it has beendeposited at least around the core during cabling and wherein it hasbeen insinuated between the strands and/or between the constituentwires.